The present invention relates to a closed loop type proportional electromagnetic valve for hydraulic control, and, more particularly, to a closed loop type proportional electromagnetic valve for hydraulic control for use in shift control systems of an automatic transmission and a power steering unit of the like of a motor vehicle, with the valve sliding spool valves controlled by an electromagnet wherein a resonance check structure for the sliding spool valves is improved.
A closed loop type proportional electromagnetic valve for hydraulic control includes a hydraulic control solenoid valve which represents an important device in the hydraulic control system for the motor vehicle, with the hydraulic control solenoid valve being adapted to be employed for controlling an automatic transmission and a power steering unit of the like in a motor vehicle.
In automatic transmission system for the motor vehicle, the throttle sensor detects the opening degree of the accelerator, and the signals from the various sensors are processed through a microprocessor. The hydraulic control solenoid valve, receiving an appropriate signal from the driver controls the main regulator and varies the line pressure fluid, with the line pressure fluid being transmitted to a friction element through the shift solenoid. By this regulated line pressure fluid, the torque to the wheels can be smoothly transmitted.
In, for example, U.S. Pat. No. 4,250,922, a conventional closed loop type proportional electromagnetic valve for hydraulic control for use in connection with an automatic transmission system of a motor vehicle is proposed.
The electromagnetically operated control valve assembly or the hydraulic control solenoid valve includes an electromagnet and a valve unit. The end portion of a plunger which projects into a valve sleeve forms a sliding spool of the valve unit. The inlet port is connected to the main line pressure fluid of the shift control system of a motor vehicle, whereas, the outlet port leads to the conduit supplying a regulated line pressure fluid to the coupling members of the automatic transmission. The exhaust port opens in a pressure free space.
However, with the above proposed structure, the electromagnetically operated control valve assembly, because the flow of the line pressure fluid flowing from the inlet port is changed by the portion of the sliding spool valves, the force of the line pressure fluid is applied to the sliding spool valves in their axial direction so that the balance of forces becomes unstable and therefore the resonance phenomenon of the movable parts is generated.
When the direction of the flow of the line pressure fluid flowing in from the inlet port is changed by the portion of the sliding spool valve, the line pressure fluid force is applied to the direction of the armature. This force makes the balance mentioned above unstable and produces the resonance phenomenon. This resonance phenomenon occurs over an entire stroke of the movement of the sliding spool valves. The self-excited vibration or the resonance phenomenon of the sliding spool valves results in a number of problems.
The resonance phenomenon occurs in the small control current domain, namely, in the relatively high output-pressure domain. Then the valve of the output-pressure varies largely and the line pressure fluid control becomes impossible.
The resonance frequency of the hydraulic control solenoid valve is governed by the specific frequency of a system having a single degree of freedom and is determined by the weight of the movable parts and a spring constant of the spring, usually 30 - 100 Hz. It is functionally impossible to change the weight of the movable parts and the spring constant of the spring substantially.
In for example, Japanese Patent Publication No. 2217/1974, a method for preparing or adding a damping action is proposed in order to solve the unstable balance of forces and the resonance phenomenon of the movable parts of the hydraulic control solenoid valve for the motor vehicle.
In above Japanese patent publication, an electromagnetically operated control sliding spool valve or the hydraulic control solenoid valve includes a movable armature of the plunger having a hole therethrough, with the through hole being provided in the movable armature providing the damping effect to the movable armature of the plunger.
However, to realized this method, a chamber for a movable armature of the plunger must be fully filled with line pressure fluid. Further, because the damping effect is obtained by a clearance between the electromagnetic coil portion and the periphery of the movable armature and the dimensions of the through hole of the movable armature, the structure of the plunger portion becomes complicated and further the dimensions on the design of the plunger portion which are necessary to determine an attraction force are restricted.
An object of the present invention resides in providing a closed loop type proportional electromagentic valve for hydraulic control wherein the resonance phenomenon of the sliding spool valves can be prevented.
Another object of the present invention resides in providing a closed loop type proportional electromagnetic valve for hydraulic control wherein the dimensions on the design of the plunger are not restricted.
In accordance with the present invention a closed loop type proportional electromagnetic valve for hydraulic control is provided which includes a coil case, a body fixed to the coil case, a coil provided the coil case, a spring provided the coil case, a plunger provided the coil case, with the coil being provided in such manner that the coil attracts the plunger against the tension of the spring at the time of excitation. A shaft is provided in the body in such a manner that the shaft is freely shiftable therein, a first sliding spool valve and a second sliding spool valve are provided with the shaft. An inlet, exhaust and outlet port are provided in the body, with the first sliding spool valve and the second sliding spool valve closing and opening the inlet port and the exhaust port. The outlet port is communicated with the shaft between the first sliding spool valve and the second sliding spool valve, and a feedback chamber is provided in the body and is communicated with the shaft through a feedback channel. The feedback chamber has a larger diameter than a diameter of the first sliding spool valve and an end portion of the first sliding spool valve opposite the second sliding spool valve protrudes into the feedback chamber.
For this purpose a damping means is fixed to the end portion of the first sliding spool valve and is positioned in the feedback chamber so as to form a constricted part of a flow passage formed between an inside wall of the feedback chamber and a peripheral surface of the damping means.
The ratio of areas of a cross-sectional area of the constricted part of the flow passage and a side surface area of the damping member is defined so as to be between a ratio of areas of an upper limit of the practical response time and a ratio of areas of less than a resonance domain.
A closed loop type of proportional electromagnetic valve for hydraulic control of the present invention facilitates prevention of the resonance phenomenon of the sliding spool valves and at the same time removes restrictions on the design from the dimensions of the plunger.
The hydraulic control solenoid valve for the motor vehicle has a pair of sliding spool valves. According to the results of the experimentation and analysis, it has been determined that the addition of the damping coefficient (C.sub.d) acted on the sliding portion of the sliding spool valve is available to stabilize the behavior of the sliding spool valves.
For the addition of the damping coefficient (C.sub.d), it has been determined that it is possible to provide a damping mechanism or a damping means with the end portion of one of the sliding spool valve positioned in the feedback chamber which is filled with the line pressure fluid or the pressure oil. The damping mechanism or the damping means gives the damping effect to the sliding spool valves and also functions as the stabilization of the behavior of the sliding spool valves.